Add docs.

src/app/shared/_pipes/safe.pipe.spec.ts

@@ -1,8 +1,12 @@
 import { SafePipe } from './safe.pipe';
+import {DomSanitizer} from '@angular/platform-browser';
+
+// tslint:disable-next-line:prefer-const
+let sanitizer: DomSanitizer;
 
 describe('SafePipe', () => {
   it('create an instance', () => {
-    const pipe = new SafePipe();
+    const pipe = new SafePipe(sanitizer);
     expect(pipe).toBeTruthy();
   });
 });